The amino acid selenomethionine has been used in x-ray crystallography as a label to determine the phase in place of heavy metal derivatives. However, in some cases, the selenomethionine-containing protein crystals give useless diffraction patterns. One hypothesis for this phenomenon is that selenomethionine is oxidized more easily than methionine. To examine this hypothesis, we have used matrix-assisted laser desorption mass spectrometry to determine the oxidized products generated by hydrogen peroxide oxidation. A new oxidized product was found in that study. To better characterize the sensitivity to oxidation for selenomethionine residue in peptide, we employed reverse-phase HPLC to quantitatively analyze the oxidized products and in-line HPLC/ESI mass spectrometry to identify the products. By this means, we have characterized the oxidation properties of selenomethionine residues in peptides. A paper describing this work will be prepared shortly.